# hygrometer question

#### PotterWatch

##### My Patronus is a Chicken
11 Years
I bought a digital hygrometer today (only one I could find). It shows temp and humidity. I followed the directions I found online for calibrating it though I used kosher salt rather than table salt (does that matter?). It has been reading steadily at 68% for the last couple hours. It doesn't have a way to adjust it, so I will just have to calculate for that difference as I go. It has been just over eight hours since I put it in the bag with the salt/water. Is it common for it to be off that much?

I am not familiar with the calibration you are talking about, could you provide a link? I would like to check mine, then we can compare.

Yes, that's very common. Just make a note--you can attach a piece of tape to the hygrometer itself with notation so you can see at a glance how much to add. Glad you got it calibrated!

Mine was off by 7 degrees too.

OK, you science guys, explain to me how this works.

Quote:
How to Calibrate a Hygrometer
by Lianne McLeod, DVM

To calibrate a hygrometer you will need:
· 1/2 cup table salt
· approximately 1/4 cup water
· coffee cup
· hygrometer
· large re-sealable freezer bag
1. Place 1/2 cup of salt in the coffee cup, and add the water. Stir for a bit to totally saturate the salt (the salt won't dissolve, it will be more like really wet sand).
2. Place the salt/water mix in a re-sealable plastic bag, along with the hygrometer, and seal the bag. Note: make sure none of the salt/water mix comes in direct contact with the hygrometer.
3. Set this bag aside at room temperature for 8-12 hours, in a location where the temperature is fairly constant.
4. After 8-12 hours, check the reading of the hygrometer. It is best to read it while still in the bag.
The relative humidity in the sealed bag with the salt/water mix should be 75 percent (mine read about 72 percent).
5. For adjustable hygrometers, adjust to read 75 percent. You will have to do this very quickly, or remember how much you need to adjust the setting (e.g. mine read 72 percent rather than 75 percent, so I would need to adjust the dial up 3 percentage points).
In my example: after sitting in the bag, my hygrometer read 72 percent, when it should have read 75 percent -- a difference of 3 percent. I now add 3 percent to the readings I take on the hygrometer (e.g. in a tank) to get the actual relative humidity.
Remember: always give a hygrometer about 2 hours to stabilize before taking a reading, as changes in the relative humidity may take a while to register accurately on a hygrometer.

Mine was off by 10 when calibrating with salt. I don't think it is that uncommon with the \$20 digitals.
When calculating at different humidity percentages keep in mind that it is not a direct addition. For example, mine read 65% when it should have read 75%. Thats a factor of 115% that needs to be added to the 65% reading. So if I am reading 35% on my meter the actaul is 40.25% (35 X 1.15 correction factor).

OK, you science guys, explain to me how this works.

From wikipedia:

"The critical relative humidity (CRH) of a salt is defined as the relative humidity of the surrounding atmosphere (at a certain temperature) at which the material begins to absorb moisture from the atmosphere and below which it will not absorb atmospheric moisture.

When the humidity of the atmosphere is equal to (or is greater than) the critical relative humidity of a sample of salt, the sample will take up water until all of the salt is dissolved to yield a saturated solution. All water-soluble salts and mixtures have characteristic critical humidities; it is a unique material property."​

Quote:
Kosher is better because it is pure NaCl. Table salt has additives such as iodine and anticaking chemicals.

Quote:
From wikipedia:

"The critical relative humidity (CRH) of a salt is defined as the relative humidity of the surrounding atmosphere (at a certain temperature) at which the material begins to absorb moisture from the atmosphere and below which it will not absorb atmospheric moisture.

When the humidity of the atmosphere is equal to (or is greater than) the critical relative humidity of a sample of salt, the sample will take up water until all of the salt is dissolved to yield a saturated solution. All water-soluble salts and mixtures have characteristic critical humidities; it is a unique material property."

Burbs, thanks for posting that. That could make a BIG difference in the original post. Mine was 70% rather than 75% so I was just quickly adding 5% as suggested in my post above, i.e., right now my hygrometer is reading 43% or 43+5=48% the old way. However, doing it correctly 75/70= 1.07 , so 43 x 1.07 = 46.01% NOT 48%. That incorrect difference of 2% could make an important difference when incubating eggs and deciding when/whether or not to add water.
Also, when comparing my hygrometer's percentage to another that's known to be 100% correct, I could not see mine at 43% and the correct one at 46% and simply figure 3% difference at every percentage change, e.g., if mine only read 12% humidity, 12 x 1.07 = 12.84% not 15%.
Thanks to you, I am going to keep a calculator next to my incubator and hygrometer from now on.

Last edited:

Replies
24
Views
3K
Replies
5
Views
2K
Replies
2
Views
536
Replies
5
Views
656